Plasma processing of a workpiece or semiconductor wafer, particularly dielectric etch plasma processing, typically employs carbon-containing process gases (e.g., fluorocarbon or fluoro-hydrocarbon gases) that enhance the etch selectivity of dielectric materials, such as silicon dioxide, relative to other materials such as silicon. These processes are used to treat the front (top) side of the wafer on which the microelectronic thin film structures are formed. The opposite (back) side of the wafer is typically un-patterned. One problem is that the carbon-containing process gases tend to form polymer precursors in the plasma, which can leave a polymer residue on the front side of the wafer and on the exposed portion of the backside of the wafer, and even some distance under the unexposed portion of the wafer backside. Such residues should be removed to avoid contamination of later processing steps. The polymer residues deposited on the wafer front side tend to be easily removed with plasma ion bombardment using appropriate chemistry. However, the wafer edge is beveled, and the curved surface on the backside of the wafer edge is also exposed and therefore susceptible to polymer deposition during plasma processing. The backside of the curved surface of the wafer edge is shadowed from ion bombardment during plasma processing so is more difficult to remove, but can be removed in an oxygen plasma at high temperature (e.g., above 300 degrees C). Such difficult-to-remove polymer films require a post-etch polymer removal step using (for example) an oxygen-rich plasma for thorough polymer removal.
In many applications, the plasma etch process is used to form openings (e.g., trenches or contact holes) through multiple thin films on the wafer front side. Such thin film structures can include (for example) a special carbon-containing dielectric film having an ultra-low dielectric constant (ultra low-K film). The ultra low-K film is exposed in cross-section at the side wall of each trench or contact opening formed by the etch process step. Attempting to remove the back-side polymer film by heating and exposing the wafer to an oxygen-rich plasma (during a post-etch polymer removal step) will damage the ultra low-K film by removing carbon from it. In semiconductor structures having 60 nm features sizes (or smaller), such damage to the ultra-low K film is permitted only to a depth of about 3 nm beyond the exposed surface (e.g., 3 nm beyond the sidewall of the opening). In contrast, the polymer film deposited on the wafer backside edge is about 700 nm thick. It is generally difficult if not impossible to avoid damaging the ultra low-K (ULK) film beyond the permissible 3 nm depth while exposing the wafer to an oxygen-rich plasma of a sufficient density and for a sufficient time to remove 700 nm of polymer from the backside of the wafer edge or bevel. The required polymer-to-ULK film etch selectivity (over 200:1) for such a polymer removal process in general cannot be maintained reliably in conventional processes.
In conventional plasma reactor chambers, the wafer support pedestal includes an annular collar surrounding the edge of the wafer. Such a collar tends to shield the wafer edge, but cannot be sufficiently close to the wafer edge to prevent polymer deposition on the backside of the wafer edge. This is because some finite gap between the wafer edge and the collar is required to accommodate variations in the robot wafer placement and tolerance stack-up. Moreover, the wafer edge-to-collar gap tends to increase as successive wafers are etched in the chamber, since the collar is (typically) formed of a process-compatible material (e.g., quartz, silicon or silicon carbide) that is gradually etched away during plasma etch processing of successive wafers. Therefore, it has seemed inevitable that unwanted polymer is deposited on the wafer, including the backside edge of the wafer.
The foregoing problems might be avoided by using a rich mixture of oxygen in the plasma during the initial etch process. However, this approach is not practical if the thin film structure on the wafer includes an ultra-low K film that is exposed on a sidewall of an etched opening. Such a rich oxygen mixture in the etch plasma would cause unacceptable damage to the ultra-low K film.
There is a need for a way of removing polymer from the backside of the wafer (i.e., the backside of the wafer edge) without harming or damaging any low-K film layers in thin film structure.